It is a common practice in the pressure swing adsorption industry to separate a mixture of gases by adsorptive techniques to produce a single high purity gas stream. However, some of the gas mixtures contain gases, two or more of which are of commercial value (e.g., H.sub.2 /CH.sub.4, CO.sub.2 /CH.sub.4, CO.sub.2 /N.sub.2, N.sub.2 /O.sub.2, H.sub.2 /N.sub.2 /CH.sub.4 /CO/CO.sub.2, H.sub.2 /CH.sub.4 /C.sub.2 H.sub.6 /CO.sub.2, etc.). For these mixtu res, it is therefore desirable to recover two gas products. The less strongly adsorbed gas species are referred as the first gas product stream in the following discussion. The more strongly adsorbed species are referred to as the second gas product stream in the following discussion.
U.S. Pat. Nos. 3,797,201; 4,578,089; 4,599,094; and 4,539,020 (Re. 32,590) among others describe pressure swing adsorption (PSA) processes to produce a single high purity gas from a multicomponent gas mixture. The most strongly adsorbed species is produced at ambient pressure. These PSA processes achieve either high purity or high recovery of the single product gas.
U.S. Pat. No. 4,592,860 describes a PSA process to obtain a mixture of H.sub.2 and N.sub.2 (Ammonia Synthesis Gas) from a multicomponent feed gas mixture obtained from a steam-methane reformer off-gas plant.
U.S. Pat. No. 3,751,878 describes a PSA process to separate methane and carbon dioxide. The process has the following four steps: adsorption at high temperature (&gt;300.degree. F.), high pressure carbon dioxide rinse, depressurization, and repressurization. The process is expected to be energy intensive due to the high temperature adsorption step and the need to compress the depressurized gas for the high pressure CO.sub.2 rinse step.
U.S. Pat. No. 4,077,779 describes a PSA process for methane-carbon dioxide separation by the following six steps: adsorption, high pressure carbon dioxide rinse, depressurization, low pressure methane purge, evacuation, and repressurization. The process has high power consumption due to high pressure CO.sub.2 rinse and low methane recovery due to the methane purge step. Carbon dioxide purity is also expected to be low.
U.S. Pat. No. 4,000,990 describes an integrated temperature swing adsorption (TSA-PSA) process to purify landfill gas. The PSA portion of the process has the following four steps to separate the binary methane-carbon dioxide gas mixture: adsorption, depressurization, evacuation, and repressurization. Methane recovery is expected to be low since most of the depressurized gas is vented to the atmosphere.
U.S. Pat. No. 4,770,676 describes two PSA processes to separate the binary methane-carbon dioxide gas mixture. The first PSA process has five steps: adsorption, high pressure CO.sub.2 rinse, depressurization, evacuation, and repressurization. Product methane is of high purity (98.sup.+ %) and is obtained at high recovery (.about.99%). Carbon dioxide product is also of high purity (.about.99.sup.+ %) and is obtained at high recovery (.about.98+%). However, the PSA process consumes a significant amount of power due to high pressure CO.sub.2 rinse step. The second PSA process described in U.S. Patent application #4,770,676 has four steps: adsorption, depressurization, evacuation, and repressurization. The depressurized gas is recycled to increase the recovery of methane and carbon dioxide gas streams. This PSA process consumes less power than the five step process but results in lower methane recovery (.about.97.sup.+ %) at high methane purity (.about.98.sup.+ %). Also, the carbon dioxide purity (.about.96.sup.+ %) and recovery (.about.97.sup.+ %) are lower than obtained by the five step process.
The drawbacks of the prior art have been overcome by the present invention by the unique utilization of the various PSA process steps including a low pressure purge with one or more, more strongly adsorbed gas components and the recycle of purge effluent and depressurization gas to the feed gas stream being introduced into the process for adsorptive separation. Unlike the prior art, the present invention results in the recovery of two gas products at high purity and high recovery from a mixture of gases, the one or more, less strongly adsorbed gas components are produced at high purity, high recovery and at the elevated adsorption pressure. The one or more, more strongly adsorbed gas components are produced at high purity, high recovery and at ambient pressure conditions. The benefits and the particulars of the present invention will be set forth in greater detail below.